Optical disk pickup using current mode signal exchanges and systems and methods using the same

ABSTRACT

An optical disk pickup system includes an array of photodiodes  101  for converting photons reflected from an optical disk into a plurality of electrical signals each representing a channel. Driving circuitry  407, 408  drives at least one of the electrical signals as a current across a conductor of a flexible cable  403.  A low impedance load  404  converts the electrical signal driven across the conductor as a current into a voltage for further processing.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The following co-pending and co-assigned application containsrelated information and is hereby incorporated by reference: Ser. No.08/956,569 (Attorney Docket No. 0746-MS]), entitled “SYSTEMS AND METHODFOR CONTROL OF LOW FREQUENCY INPUT LEVELS TO AN AMPLIFIER ANDCOMPENSATION OF INPUT OFFSETS OF THE AMPLIFIER” filed Oct. 23, 1997;

[0002] Ser. No. 09/703,315 (Attorney Docket No. 0924-MS [2836-P082US]),entitled “OPTICAL DISC. PICKUP SYSTEM USING CURRENT DIVISION SIGNALTRANSMISSION AND METHODS AND OPTICAL DISK SYSTEMS USING THE SAME, filedOct. 31, 2000, granted Jul. 9, 2002 under U.S. Pat. No. 6,418,110;

[0003] Ser. No. 09/282,121 (Attorney Docket No. 0926-MS [2836-P083US]),entitled “CIRCUITS AND METHODS FOR EXCHANGING SIGNALS IN OPTICAL DISKSYSTEMS AND SYSTEMS USING THE SAME”, filed Mar. 31, 1999, currentlypending;

[0004] Ser. No. 09/282,840 (Attorney Docket No. 0927-MS [2836-P084US]),entitled “CIRCUITS AND METHODS FOR GAIN RANGING IN AN ANALOG MODULATORAND SYSTEMS USING THE SAME”, filed Mar. 31, 1999,currently pending;

[0005] Ser. No. 09/282,841 (Attorney Docket No. 0951-MS [2836-PO90US]),entitled “A FLEXIBLE INTERFACE SIGNAL FOR USE IN AN OPTICAL DISK SYSTEMAND SYSTEMS AND METHODS USING THE SAME”, filed Mar. 31, 1999, currentlypending; and

[0006] Ser. No. 09/282,849 (Attorney Docket No. 0971-MS [2836-P091US]),entitled “SERVO CONTROL LOOPS UTILIZING DELTA-SIGMA ANALOG TO DIGITALCONVERTERS AND SYSTEMS AND METHODS USING THE SAME” filed Mar. 31, 1999,currently pending.

BACKGROUND OF THE INVENTION

[0007] 1. Field of the Invention

[0008] The present invention relates in general to optical disk systemsand in particular to an optical disk pickups using current mode signalexchanges and systems and methods using the same.

[0009] 2. Description of the Related Art

[0010] Optical disks have been used for many years for the mass storageof digital data. Some well known examples of optical disks includedigital audio compact disks (CD-DAs), compact disk read-only memories(CD-ROMs) and digital video disks (DVD-RAMs, −ROM, +RW, −RW, CD-R,CD-RWs). Essentially, digital data is stored on a plastic disk with areflective surface as a series of pits and land in the reflectivesurface (land). During playback, a beam of light is directed to therotating reflective surface and the intensity of the photons reflectedfrom the pits and land are measured. A modulated electrical signal isgenerated that can be processed and the data stored on the diskrecovered.

[0011] A basic configuration for the read (playback) mechanism hasdeveloped over a number of years. This configuration includes a pickupor sled which is movable so that a laser, a lens, and array ofphotodiodes can be positioned directly over the data being read off ofthe disk. As the disk turns, the photons from the laser are reflectedoff the pits and land and received by the photodiodes which generateelectrical signals having a current that is proportional to photondensity.

[0012] The multiple signals output from the photodiodes represent bothdata detection and servo alignment information. The summation of thehigh speed data channel signal, which may be composed of the signalsA+B+C+D from an astigmatic photodiode array, results in a compositesignal with relevant information between approximately 10 KHz and 60 MHzfor current DVD players. Servo information contained in these signalshowever, is at frequencies less than 1 MHz down to dc (for currentspindle rotation rates of <6000 RPM). Because of these informationrates, the data channel signal is sometimes AC-coupled to the datadetection and summation circuitry mounted on an accompanying stationarycircuit board. Otherwise, some degradation of the dynamic range must beaccepted due to the dc content of the incoming signal.

[0013] The typical current signal generated by a photodiode is on theorder of 1 uA. Transferring this signal directly down a flexible cableto the stationary circuit board can seriously degrade the signal tonoise ratio due to magnetic or electrical interference. Hence,transimpedance amplifiers, which convert the current from the photodiodearray into a voltage for driving the cable, are mounted in the pickup tominimize noise and interference effects. The data detection, errorcorrection, and servo systems are kept off of the pickup primarily toreduce the physical size and mass of the sled.

[0014] One of the primary concerns about transferring data across theflexible cable as a voltage is maintaining a good signal to noise ratio,in the presence of interference. A good signal to noise ratio can beachieved by insuring that the output of the pickup electronics aredriven across the flexible cable using a sufficiently high supplyvoltage. Notwithstanding, it would be desirable to be able to reduce thesupply voltage to save power; however, to do so would reduce theamplitude of the signals being transmitted across the cable and hencereduce the signal to noise ratio. What is needed therefore are methodsand circuitry which maintain the signal to noise ratio for signals beingtransmitted across the flexible cable, even if the supply voltage isreduced.

SUMMARY OF THE INVENTION

[0015] An optical disk pickup system is disclosed including an array ofphotodiodes for converting photons reflected from an optical disk into aplurality of electrical signals each representing a channel. Drivingcircuitry is provided for driving at least one of the electrical signalsas a current across a conductor of a flexible cable. A low impedanceload converts the electrical signal driven across the conductor as acurrent into a voltage for further processing.

[0016] The present inventive teachings provide a number of advantagesover the prior art. Among other things, by driving the flexible cableusing current rather than a voltage, the voltage on the supply rails canbe reduced significantly. Specifically, the voltage headroom can bereduced without substantially affecting the signal to noise ratio.Moreover, data and servo control signals being transmitted can be easilysummed to reduce the number of conductors required on the systemflexible cable, also without exceeding the available voltage headroom.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

[0018]FIG. 1 is a conceptual diagram of an exemplary personal computerbased optical disk playback system;

[0019]FIG. 2 is a detailed functional block diagram of the data pathshown in FIG. 1;

[0020]FIG. 3 is a diagram showing further detail of the servo controlpath shown in FIG. 1; and

[0021]FIG. 4A is a more detailed functional block diagram of a currentmode signal transmission/reception system suitable for use in the systemof FIG. 1;

[0022]FIG. 4B illustrates an alternative current mode signaltransmission/reception system; and

[0023]FIG. 5 depicts the summing of currents to generate compositesignals for reducing the number of conductors required in the flexiblecables of FIGS. 4A and 4B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The principles of the present invention and their advantages arebest understood by referring to the illustrated embodiment depicted inFIGS. 1-4 of the drawings, in which like numbers designate like parts.

[0025]FIG. 1 is a conceptual diagram of an exemplary personal computer(PC) based optical disk playback system including a drive managerintegrated circuit (IC or “chip”) 100 embodying the present inventiveconcepts. It should be recognized however that IC 100 can also be usedwith CD or DVD players and DVD RAM systems. In addition to chip 100, thesystem also includes optical pickup 101, including the requisite laser,photodiode array and transimpedance amplifiers, and the power amplifiers102 and motors & actuators 103 which control the player spindle 104rotation and pickup 101 movement and alignment. In the preferredembodiment, drive manager chip 100 embodies decoding circuitry forprocessing data from either DVD-ROM, CD-ROM or CD-DA optical disks.

[0026] There are two principal processing paths, one each for the servoand data channels, the inputs of which are driven by the transimpedanceamplifiers on optical pickup 101. The servo path is shown generally at300 and the data path generally at 200. Each of these paths will bediscussed in further detail below in conjunction with FIGS. 3 and 2respectively. The output of the data channel is passed through ECC andDecoder 105 for additional processing such as error correction andcontent descrambling.

[0027] Local control is implemented by microcontroller 106 throughmicrocontroller interface 107. Typically, local microcontroller 106 isuser supplied for maximum flexibility and generally provides theinstructions directing the on-board processors and error correctioncircuitry.

[0028] Chip 100 additionally communicates with a host processor 108 viaan ATAPI bus interface 109 and ATAPI bus 110, in the case of a PC-basedsystem. The host performs the actual processing of the audio/videoinformation or data retrieved from the disk after error correction andbuffering by chip 100. Among other things, the host performs audio andvideo MPEG decoding and generates the corresponding user interface.Buffers (DRAM) 111 support error correction functions and the streamingof data from chip 100 to host 108.

[0029] Referring to FIG. 2 which is a detailed functional block diagramof data path 200, attenuators 201 are used in the preferred embodimentto protect the inputs of the following VGAs from damage from anyover-voltages produced by the pickup. Offset controls 203 a and 203 ballow the digital offset control loop discussed below to respond to dcand low frequency baseline offsets in attenuators 201 and VGAs 202.

[0030] Data channel summation and variable gain amplifier (VGA)circuitry 202 add one or more signals from the transimpedance amplifierson pickup 101 to form a composite data signal (e.g., A+B+C+D).Alternatively, the signal addition may be done right on pickup 101,either electrically or optically. The VGA gain is controlled byautomatic gain control loops, also discussed below.

[0031] A low pass filter (LPF) 204 provides anti-aliasing for flashanalog to digital converter 205. A digital moving average of the outputof ADC 205 is taken and filter 206 applied to reject noise andinterference in the Nyquist bandwidth, as well as perform a decimation.It should be noted that any one of a number of other types of filterscan be used to achieve the same result. The decimating filter 206 canalso be used to lower the effective sampling rate of the data forsubsequent digital data processing. The data is then digitally equalizedusing a multiple-tap finite impulse response (FIR) filter 207 adjustableto differing data rates and disk reflectivity. Advantageously, thefront-end analog circuits are simplified since data is immediatelydigitized and the necessary equalization is performed digitally.

[0032] Automatic offset control is implemented by the loop includingenvelope detectors 208, offset controls 209 and DAC 210. Envelopedetectors 208 detect both the top and bottom envelopes of the high speeddata signal. These envelopes are summed to produce an error signal whichis passed through an offset loop compensation filter within offsetcontrol block 209 and integrated. The output of the loop compensationfilter is converted to analog form by DAC 210 and summed with the outputof LPF 204.

[0033] Gain control loop 211 also takes the difference between theamplitudes of top and bottom detected envelopes and subtracts apre-programmed gain value. A gain loop compensation filter integratesthe results and produces a linearized signal which is converted by DAC212 to analog form and passed to VGAs 202 to adjust the signal gain.

[0034] An interpolating digital phased-locked loop (DPLL)213 retimes thedata after ADC sampling and digital equalization. DPLL 213 operates onsampled amplitudes and generally includes a digital phase errordetector, digital loop compensation filter, and digital frequency tophase integrator (digital VCO). Variable delay filter 214 interpolatesthe asynchronous digital samples to ideal synchronously sampled samplesat the front of the DPLL. The phase detector then generates an errorsignal using a stochastic process which compares the incoming data withideal target sampling values without noise. The error signal ismultiplied by the derivative of the target data to produce phase errorestimates. The loop compensation filter performs a proportionalintegration and the result is sent to variable delay filter 214 toadjust the delay and correct for phase errors.

[0035] Advantageously, digital PLL 213 allows the ADC and equalizer tooperate at a fixed asynchronous sample rate to the data.

[0036] Asymmetry control circuitry 215 includes a control loop whichcorrects the read errors from the optical pickup. The errors aredetected using either the slicer duty cycle or zero crossing errors. Theerrors are then scaled and integrated by a compensation filter and theresulting compensation signal summed at the input to variable delayfilter 214.

[0037] The retimed data is then processed by a maximum likelihoodsequence detector 216. The partial response equalization target assumedin this detector is G(D)=1+D+D²+D³. Other targets may be used inalternate embodiments.

[0038] The output of sequencer 216 is synchronized by framesynchronization circuitry 217 and then passed to Run Length Limit(“RLL”) decoder 218. RLL code embedded in the disk is used as anindication of disk defects. Generally, a state machine checks forviolation of the RLL code “k-constraint” and failures in synchronizationand then causes the data channel to “coast” through the defect and thenresynchronizes the data stream.

[0039] Automatic Zone Control (AZC) logic (not shown) takes advantage ofthe digital nature of the data channel by initializing subsystems basedon data rate. For example, the tap weights and tap spacing of thedigital equalizer are set to correspond to one of six incoming datarates. Similarly, the loop coefficients, and hence the loop dynamics, ofinterpolating digital PLL 213 are controlled by the AZC logic.

[0040] In sum, the data channel is a bandpass system with signals in the10 kHz to 60 MHz range. The signal spectrum below 10 kHz is either servoinformation or external dc offsets from the pickup electronics. Thepresence of this information reduces the dynamic range of the datachannel. Using an off-chip ac coupling capacitor would reduce the dcoffset but blocks the low frequency servo information. Instead, the dcsignal is brought on-chip and a control loop performs the effective accoupling for the data channel. Not only are external coupling capacitorsunnecessary, but defect detection by the downstream digital processingcan freeze this control loop when a defect is reached, unlike an accoupled system where the baseline wanders. The offset and AGC loops arealso frozen until data transitions are detected.

[0041] Co-pending and co-assigned application Ser. No. 08/956,567(Attorney Docket No. 0746-MS), entitled “SYSTEM AND METHOD FOR CONTROLOF LOW FREQUENCY INPUT LEVELS TO AN AMPLIFIER AND COMPENSATION OF INPUTOFFSETS OF THE AMPLIFIER” filed Oct. 23, 1997 contains relatedinformation and is hereby incorporated by reference.

[0042] Decoder block 105 (FIG. 1) manages the flow of data between thedata channel and external DRAM buffer 111 and manages PC host ATAPIinterface 109. The ECC circuitry performs realtime ECC correction forDVD data and layered ECC correction for CD-ROM data. Additionally 8-14demodulation is provided for DVD data and EFM demodulation for errorcorrection and deleaving of CD-DA and CD-ROM data. A burst cutting area(BCA) decoder is built-in chip 100 for DVD-ROM applications. DVDNavigation Play for DVD player operations is supported along withContent Scramble System circuitry for descrambling DVD data which hasbeen scrambled under the Content Scramble System. The error correctionand decoding functions are supported by on-chip SRAM.

[0043] As indicated above, the second principal signal path of the chip100 controls servo operation and is shown generally at 300 in FIG. 1 andin further detail in FIG. 3. The integrated servo system operates fourcontrol loops: focus, tracking, sled, and spindle, using an internalservo control processor requiring little external microcontrollerintervention.

[0044] Servo data is received from each of the six photodiodes 101 andthen amplified by six VGAs 301. As a result, the following ADCs 302 onlyrequire 60 dB of dynamic range, because servo VGAs 301 boost the inputsignal by as much as 28 dB. VGAs 301 also incorporate low pass filtering(LPF) for anti-aliasing. Preferably three pole filters are used with onepole in front of the VGAs and two poles after the VGAs.

[0045] Analog to digital conversion is done immediately after low passfiltering such that the analog/digital boundary is as close to the inputas possible. An input sampling frequency of 24 MHz (generated on-chip bysample rate generator 303) and a third order delta-sigma modulatorreduce digital filter group delay inside the servo loop.

[0046] Servo data processing is performed by on-board servo controlprocessor (SCP) 304, which receives its instruction set from the userselected local microcontroller 106 through interface 107 and RAM 305.

[0047] Unlike CD systems, DVD servo systems use differential phasedetection (DPD) between the photodiode signals D1,D2 (D1=A+C, D2=B+D)for track following and track counting. A digital adaptive dual armcorrelator (ADAC) is implemented. This is superior to the conventionalDPD methods based on a simple phase detector and analog filters.

[0048] Analog control signals are transmitted to power amplifiers 102through DAC array 306 and spindle control 307.

[0049] According to the principles of the present invention, signals aretransmitted across the flexible cable in an optical disk system ascurrents rather than voltages. The voltage signals can then be recoveredat the receiving end using a low impedance load. By using current, widedynamic range can be achieved without sacrificing the signal to noiseratio. One embodiment of these principles is depicted in FIG. 4A.

[0050]FIG. 4A is a more detailed functional block diagram of a currentmode signal transmission/reception system 400 according to the inventiveconcepts. The current output from the corresponding diode 401, which isapproximately 1-10 μA, is converted into a voltage and amplified by atransimpedance amplifier consisting of an operational amplifier 405 anda feedback resistor 406. The feedback resistor may for example be on theorder of 4 k ohms. The output voltage VA from operational amplifier 405may be on the order 500 to 600 mV.

[0051] The output from the transimpedance amplifier is reconverted tocurrent and amplified by a transconductance amplifier 407.Transconductance amplifier 407 outputs the signal at approximately 100μA for transmission across a corresponding conductor of the flexiblecable 403. Prior to transmission, the currents representing the variousdiodes may be summed to generate composite signals for transmission on areduced number of conductors. For example, the currents output from thetransconductance amplifiers associated with photodiodes A, B, C, D, E,and F may be summed to produce a composite signal for delivery to servocontrol channel 300.

[0052] An alternative configuration is shown in FIG. 4B. Here, thecurrent output from the given photodiode 401 is directly multiplied by acurrent multiplier 408 and transmitted across flexible cable 403. WhileFIG. 4B depicts single-ended transmission, differential transmission canbe used equally as well. Again, the currents generated by the multiplier408 corresponding to different diodes may be summed to generatecomposite signals for transmission to fixed circuit board 404.

[0053]FIG. 5 illustrates the summing of currents to generate compositesignals for reducing the number of conductors required in the flexiblecables. Adding currents also decreases the required headroom that wouldnormally be required if voltages were summed. Here, each diode 401 iscoupled to a current driver 501. Drivers 501 may, for example, beconstructed as the transimpedance—transconductance amplifier pairdiscussed above or a current multiplier. In any event, the currentsproduced from the electrical signals output from the correspondingdiodes are summed by summer 502. In this example, the outputs fromdiodes A-F are being summed together to produce a composite servocontrol signal at output SERVO OUT. This signal is sent to the input ofservo channel 300, with summation at the fixed circuit board no longerrequired. Again, a low impedance load can be used to convert thereceived current signal to a voltage signal to continue data processing.

[0054] Although the invention has been described with reference to aspecific embodiment, these descriptions are not meant to be construed ina limiting sense. Various modifications of the disclosed embodiments, aswell as alternative embodiments of the invention will become apparent topersons skilled in the art upon reference to the description of theinvention. It should be appreciated by those skilled in the art that theconception and the specific embodiment disclosed may be readily utilizedas a basis for modifying or designing other structures for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

[0055] It is therefore, contemplated that the claims will cover any suchmodifications or embodiments that fall within the true scope of theinvention.

What is claimed:
 1. An optical disk pickup system comprising: aphotodiode for converting photons reflected from an optical disk to acurrent; a transimpedance amplifier for converting the current from thephotodiode into a voltage signal; and a transconductance amplifier fordriving a conductive line with a current generated from the voltagesignal.
 2. The optical disk pickup system of claim 1 wherein thetransimpedance amplifier comprises an operational amplifier and aresistive feedback loop.
 3. The optical disk pickup system of claim 1and further comprising a low impedance load disposed in receivingcircuitry coupled to the conductor for reconverting the current drivenon the conductor into a voltage.
 4. The optical disk pickup system ofclaim 1 and further comprising a summer for summing the current from thetransconductance amplifier with two or more currents generated from theoutput of two or more photodiodes.
 5. The optical disk pickup system ofclaim 1 wherein the conductor comprises one of a plurality of conductorsof a flexible cable.
 6. An optical disk pickup system comprising: aphotodiode for converting a plurality of photons reflected from anoptical disk to a current; and a current multiplier for increasing thecurrent from the photodiode to drive a conductor.
 7. The optical diskpickup system of claim 6 and further comprising a low impedance loaddisposed in receiving circuitry coupled to the conductor forreconverting the current driven on the conductor into a voltage.
 8. Theoptical disk pickup system of claim 6 and further comprising a summerfor summing the current from the transconductance amplifier with two ormore currents generated from the output of two or more photodiode. 9.The optical disk pickup system of claim 6 wherein the conductorcomprises one of a plurality of conductors of a flexible cable.
 10. Theoptical disk pickup system of claim 6 wherein the current multipliercomprises a transimpedance amplifier for converting the current from thephotodiode to a voltage and a transconductance amplifier for driving theconductor with a current generated from the voltage output of thetransimpedance amplifier.
 11. An optical disk system comprising: anarray of a plurality of photodiodes for converting photons reflectedfrom an optical disk into a plurality of electrical signals eachrepresenting a channel; circuitry for driving at least one of saidelectrical signals as a current across a conductor of a flexible cable;and a low impedance load for converting the electrical signal drivenacross the conductor as a current into a voltage.
 12. The optical disksystem of claim 11 wherein said circuitry for driving comprises acurrent multiplier.
 13. The optical disk system of claim 11 wherein saidcircuitry for driving comprises a transimpedance amplifier forconverting a current produced by a corresponding photodiode into avoltage and a transconductance amplifier for driving the conductor witha current from the voltage output from the transimpedance amplifier. 14.The optical disk system of claim 11 wherein said circuitry for drivingand said photodiodes are disposed on a movable sled and said lowimpedance load is disposed on a fixed circuit board.
 15. The opticaldisk system of claim 11 wherein said channels comprise servo controlchannels.
 16. The optical disk system of claim 11 wherein said channelscomprise data channels.
 17. The optical disk system of claim 11 andfurther comprising circuitry for summing a plurality of said electricalsignals as currents for transmission as a single current signal acrosssaid conductor.
 18. A method of transmitting signals from an opticalpickup to processing circuitry via a flexible cable comprising the stepsof: converting photons reflected from an optical disk into an electricalsignal; driving the electrical signal as a current across a conductor ofthe flexible cable; and converting the electrical signal driven acrossthe conductor as a current into a voltage using a low impedance load.19. The method of transmitting of claim 18 comprises the step ofmultiplying a current output from said photodiode.
 20. The method ofclaim 18 wherein said step of driving comprises the substeps of:converting a current produced by a corresponding photodiode into avoltage using a transimpedance amplifier; and driving the conductor witha current generated by a transconductance amplifier from the voltageproduced by the transimpedance amplifier.